CHAPTER V. 

 THE NERVOUS SYSTEM OF THE FEOG. 



The nei-vous system consists of the following parts : 



1 . A central portion, the brain and spinal cord, which lies 



in the cartilaginous and bony tube formed by the 

 cranium and vertebral column, and which is the centre 

 where sensations are felt, and whence motor imptdses 

 causing the muscles to contract take their origin. 



2. A peripheral portion, the nerves themselves, which 



connect the central portion with the skin, sense 

 organs, muscles, viscera, etc., and serve to convey 

 sensory impulses from these parts to the brain and 

 cord, or motor impulses from the central organs to 

 the muscles. These two functions are fulfilled by 

 different nerves, which may accordingly be distin- 

 guished as (a) afferent or sensory nerves, conveying 

 impulses to the brain or cord and (6) efferent or 

 motor nerves, conveying impulses from the brain 

 or cord. 



•Fig. 12. — The nervous system of the edible frog (Rana esculenta), from the 

 ventral surface. (From Ecker. ) 



F, facial nerve ; G, ganglion of pneumogastric nerve ; He, cerebral hemi- 

 sphere ; Lc, optic tract ; Lop, optic lobe ; M, boundary between medulla 

 oblongata and spinal cord ; M 1-10, the spinal nerves ; MS, connection 

 between fourth spinal nerve and sympathetic chain ; N, nasal sac ; >^^£jgl^ 

 "PT?^ ; No, crural nerve ; o, eyeball ; S^-timik^-Ql-sym patheti c ; S 1-10, the 

 "iTmrilt''"'''' fTinrl'l ' Sp> continuation of sympathetic into liEad. 



I, olfactory nerve ; II, optic nerve ; III, motor oculi ; IV, fourth nerve ; 

 V, trigeminal and facial nerves ; Va, ophthalmic branch of trigeminal ; 

 Vc, maxillary branch of trigeminal; Vd, mandibular branch of trigeminal; 

 Ve, hyomandibular branch of facial ; Vg, Gasserian ganglion ; Vs, upper end 

 of sympathetic trunk, in connection with Gasserian gangUon ; VI, abducens 

 nerve; VII, facial nerve; VIII, auditory nerve; X, glossopharyngeal and 

 pneumogastric nerves ; '^'' rillTlin ""'"-— ">• rln^-"p harynreal : x l, ram ua. 

 posterior of glossopharyngeal ; X 3-4, branches of pneumogastric. 



